Generally, an ordinary polyether-polyol produced by cationic polymerization such as polymerization of tetrahydrofuran or copolymerization of tetrahydrofuran with any other cyclic ether has a broad molecular weight distribution. However, it is known that, when a polyether-polyol having a narrow molecular weight distribution is used for the production of polyurethane elastic fibers, polyether-ester elastomers and the like, the products may have well-balanced physical properties as compared with those from the polymer having a broad molecular weight distribution, and the products may accurately exhibit their properties in accordance with the object thereof and, in addition, there can be produced good elastomers having improved heat resistance, especially excellent dynamic physical properties such as elongation recovery and repetitive compression resistance. On the contrary, for example, polytetramethylene ether glycol obtained through polymerization of tetrahydrofuran by the use of a typical catalyst, fluorosulfonic acid, chlorosulfonic acid or acetic anhydride-perchloric acid has a broad molecular weight distribution, since the polymerization mode for it is cationic polymerization. In addition, its molecular weight distribution does not follow the rule of Gauss distribution but is biased toward the high-molecular fraction. The broad molecular weight distribution of the polymer is a long-pending question in the art. In particular, with the recent tendency toward precision industries, the request for polyether-polyol with a narrow molecular weight distribution is increasing (e.g., see JP-B 57-47687, the term “JP-B” used herein means an “examined Japanese Patent Publication”).
It has heretofore been well known that a polydisperse polymer produced through an ordinary polymerization may be fractionated into a monodisperse polymer through an ordinary liquid-liquid fractionation in a combination of an oleophilic or non-polar solvent and a hydrophilic or polar solvent. The fractionation of polyoxypropylene glycol, polyoxyethylene glycol and polytetramethylene ether glycol in cyclohexane-toluene/water-methanol with varying the ratio of water-methanol therein gives polymers of almost monodispersion (e.g., see Makromol. Chem., 41, 61 (1960) and J. Appl. Poly. Sci., 9, 467 (1965)). Also, a method of fractionating tetrahydrofuran polymer or tetrahydrofuran-alkylene oxide copolymer in cycloalkane/water-methanol is disclosed (e.g., Japanese Patent 3,352,702).
However, even though polyoxytetramethylene glycol having a narrow molecular weight could be obtained in these methods in which some organic solvents are used in combination thereof, there are still problems in that much energy is needed for solvent recovery and solvent separation may be often difficult. Still another but more serious problem is that the residual fraction that remains after the fractional extraction of the intended product having a narrow molecular weight distribution must be subjected to post treatment. Anyhow, these methods have the industrial disadvantages as above.